Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 110
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PÁLL IMSLAND
cent MgO, but appear again in the tristanites and trachytes as sporadic
small phenocrysts or microphenocrysts of more fayalitic compositions.
Kink-banded magnesian olivine (~Fo90) makes up about 50 per cent of
the wehrlite xenoliths occurring in the ankaramites. Small amounts of
olivine occur in the gabbro xenoliths, which occur in the basalts. These are
much poorer in magnesium (~Fo64) than those of the wehrlite. Other
xenoliths are free of olivine.
II. The composition of the olivines
The olivines of the rock suite range from Fo92-5i. Representative analyses
are given in Table 17. The olivines of each sample show a considerable
range in composition in most cases. In the relatively scarce rocks containing
olivine as a groundmass mineral only, the composition is homogeneous as is
the case in some of the samples containing olivine as microphenocrysts. The
compositions of the olivines of each rock are given in Fig. 71, where the Fo
content of the olivines is plotted against wt. per cent MgO of the whole rock.
The figure shows that the within-sample range of the composition may
exceed 15 per cent of the end member molecule. In the ankaramites this
compositional range usually exceeds 10 per cent. The bulk of the olivines of
the ankaramites, though, cluster around Fo90; the same composition as that
of the olivines (kink-banded) of the wehrlite xenolith occurring in the
ankaramites. No clear difference is found between the kink-banded and the
unstressed olivines. The only compositional difference may be a tendency
towards a slightly lower Fo content for the unstressed olivines. The max-
imum Fo content of the olivines is around Fo90 in all rocks containing more
than ca. 5 wt. per cent MgO. In the Mg poorer rocks the maximum Fo
content varies down to Fo5i.
Six minor- and trace elements have been determined in most of the
analysed olivines. Mn, Ca and Ni usually occur in concentrations which
may be called minor element concentrations, while Cr, A1 and Ti are much
lower; mostly below 0.1 expressed as wt. per cent oxide, and are thus better
termed trace elements. These 6 elements are plotted against Fo content of
the olivines in the variation diagrams (Figs. 72—74).
MnO in the olivines ranges from 0.1 to 2.6 wt. per cent. It increases with
increasing Fe, forming a relatively narrow trend in the Fo rich olivines but
scattering considerably in the more fayalitic ones. The most fayalitic olivines
analysed in the rock suite are Fo53_5i occurring in a trachyte lava (Jan 168).
These contain between 2.5 and 2.6 wt. per cent MnO. The olivines of the
ankaramites mostly contain between 0.1 and 0.25 wt. per cent MnO. At F076
the Mn content begins to scatter. At Fo75 e.g. the MnO values range from
0.4 to 1.0 wt. per cent. This Mn-Fe correlation is in agreement with other
observations on the behaviour of Mn in olivines, e.g. in Hawaiian ultramafic
inclusions (White, 1966), Hawaiian tholeiitic lava lake (Moore & Evans,